[Refactor]: clean up diffusion executor lifecycle (#5215)

Signed-off-by: Huang, Zeyu <11222265+fhfuih@users.noreply.github.com>

vllm_omni/diffusion/executor/abstract.py

@@ -59,7 +59,7 @@ class DiffusionExecutor(ABC):
             raise ValueError(f"Unknown distributed executor backend: {distributed_executor_backend}")
         return executor_class
 
-    def __init__(self, od_config: OmniDiffusionConfig):
+    def __init__(self, od_config: OmniDiffusionConfig) -> None:
         self.od_config = od_config
         self._init_executor()
 
@@ -68,6 +68,12 @@ class DiffusionExecutor(ABC):
         """Initialize the executor (e.g., launch workers, setup IPC)."""
         pass
 
+    @property
+    @abstractmethod
+    def is_dead(self) -> bool:
+        """Whether the executor is shut down or has failed fatally."""
+        pass
+
     @abstractmethod
     def execute_request(self, scheduler_output: DiffusionSchedulerOutput) -> BaseRunnerOutput:
         """Execute request-mode work from a scheduler output."""